STS-133: Plan under development to repair ET-137 foam cracks at the pad
As access platforms are set up for engineers to begin the removal of the hardware associated with the Ground Umbilical Carrier Plate (GUCP), a large effort to build a repair plan – one which can be conducted at Pad 39A – is being developed for the large crack (and several other smaller cracks) that were found on Discovery’s External Tank (ET-137) intertank flange region.
Discovery has completed turnaround operations, following Friday’s scrub – caused by a leak from the GUCP, which breached Launch Commit Criteria (LCC), and ultimately pushed STS-133′s launch date out to a NET (No Earlier Than) November 30 target, although the actual launch date remains under evaluation.
Work is set to begin on disconnecting the vent arm, ordnance and connections on the GUCP from Tuesday, which will lead to the removal and examination of the flight seal – one of the leading suspects for the gaseous hydrogen leak. Engineers will also take the opportunity to confirm the correct alignment of the hardware, with a misalignment also one of the likely causes of Friday’s scrub.
In preparation for this task, Discovery was safed and prepared for her extended stay at Pad 39A, with the Rotating Service Structure (RSS) moved to mate, along with the draining of her Power Reactant Storage and Distributation (PRSD) system, along with preparations for the depressurization of the orbiter’s Composite Overwrap Pressure Vessels (COPV).
An External Platform (ESP) has also been raised around the Space Shuttle Main Engines (SSMEs), which would allow for an opportunity to take a closer look at ME-3′s Main Engine Controller (MEC) during the delay.
“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (Pad-A): Significant Processing Operations: ET drain and boiloff is completed. RSS has been moved to mate position, Orbiter Weather Protection is extended. SSME ESP is in place but the SRB (Solid Rocket Booster) ESPs remain lowered,” noted the NASA Test Director (NTD) processing report (L2).
“PRSD commodities were drained and systems secured. Ordnance remains connected but is planned for safing on Wednesday morning. Hyper system COPV tanks remain pressurized so the Pad is open for scheduled work items only. MPS (Main Propulsion System) COPV tanks are vented to nominal pressure.”
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Two presentations (L2) were created for review on Monday, ahead of the GUCP work, referencing the testing facility that is likely to play a role in the mitigation process, which noted encouraging test results prior to STS-133′s scrub, when changing (increasing) the purge flowrate to reduce hydrogen concentrations during the next tanking.
There are also notes of “Significant GUCP movement” during the STS-133 leak, suggesting something was seriously wrong with the hardware during tanking (an article will be forthcoming).
“IPR-68: Engineering is working an effort to review the fault tree from previous investigations while preserving the evidence of the hardware. An ERB this morning is anticipated to establish the initial steps in troubleshooting the problem with work to start tomorrow (Tuesday),” added the NTD report.
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However, the crack on the LO2 intertank flange area of Discovery’s tank is also a large driver for the get-well plan to allow STS-133 to return to a launch posture.
“IPR-72: During the drain operations a large crack was observed on a closeout area of the External Tank. The issue developed during the de-tanking operation and was inspected on Saturday,” noted the NTD.
“Engineering took measurements and recorded observations of the area. The evaluation effort is (continuing) with discussions centering on repair options available while at the pad.”
The crack was overviewed on Monday morning by the KSC Engineering Review Board (ERB), noting the time the crack appeared and the opening forward plan.
The initial goal centers around TerraHertz and BackScatter scans of the crack in the tank’s Thermal Protection System (TPS) foam, prior to dissecting the area.
“The crack appeared at 7:08 am EDT, which was a few minutes prior to the initiation of the GUCP LH2 leak. A plan to dissect the defective area will begin no earlier than Wednesday morning,” noted ERB notes (L2).
“Non-Destructive Evaluation (NDE) using TerraHertz and BackScatter equipment will be performed in parallel to the dissection plan to determine if there is any metal substrate damage that may have caused the TPS defect to form. Access to this area is difficult, but an access platform will allow the work, described above, to occur.”
A decision on the ability to repair the tank at the pad will be made by the end of the week, with the main considerations relating to the environmental conditions required for the new foam to be sprayed on to the tank, along with the cure period once applicated.
It has already been suggested that a plan involving a hood structure – located around access platforms – would need to be devised to provide such acceptable conditions.
“On Friday, after the dissection/NDE data has been collected, the teams are expected to reconvene to determine if the ET can be repaired at the pad. Environmental conditions, such as temperature and humidity levels, must be controlled to allow a ‘spray-in-place’ plan to occur,” added the ERB notes.
“Some sort of ‘hood’ structure would have to be built around the ET/access platform to provide this environmental control. The KSC team is also working to develop a preliminary TPS R&R schedule.”
Further information is listed in a presentation, created on Monday and acquired by L2, which notes the crack is on Station Location Xt-862 and is surrounded by several other cracks.
“During STS-133 Post Drain walkdown a large crack/divot was observed in the LO2-I/T Flange TPS closeout. Location – Xt-862, +Z/-Y Quadrant – PHI angle -51deg. Timing – occurred at ~0708 hrs Local (captured by camera OTV 041),” noted the presentation.
It was also confirmed that there wasn’t any visual evidence of a foam anomaly via the photography taken of the tank before it left the Vehicle Assembly Building (VAB) – a total of three photography runs.
Engineers will likely work a modified plan to access this area of the tank, based on the pull-plug tests that were called for after a number of foam liberations from the intertank stringers several flights ago.
The pull tests have successfully added confidence that the intertank foam has the required adhesive properties to stop the foam from liberating during ascent.
“Closest Access: (Based on Intertank Foam Bond Adhesion Check): Scaffolding access to -Y “trapezoid” (high risk) areas – no reach. Stow position of -Y Pad Access Platform – adjacent,” the presentation added.
Once the issue of access has been mitigated, an opening plan to dissect the damaged foam has already been presented by the Michoud Assembly Facility (MAF) in New Orleans, who will have a Kennedy team already on station to provide support for the three final tanks – ET-137, ET-138 and ET-122 – which are all now located in Florida.
“MAF developing a Dissection Plan for defect analysis. Cut/excavate loose TPS in horizontal sections along gridlines. Document condition of separation plane (e.g., knitline debond, thin fuzzy film, etc.). Sample any exposed primed substrate for contaminants. Perform offline dissection analysis of removed sections.”
Meetings will continue through Tuesday, focusing on access, TerraHertz and BackScatter NDE scans, and refinements to the repair plan ahead of Friday’s decision on if they will be able to proceed with the repair at the pad.
(Follow L2′s STS-133 Special for up-to-the-minute engineering updates, and the forum’s STS-133 troubleshooting thread. All images via L2 presentations).